Case of the Dwindling Cloud Forest

Something is amiss in Costa Rica's famous fogged­in hilltops, and global climate change may be the cause

07-01-2000
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Bob Holmes - Photographs by Michael and Patricia Fogden

Naturalist Michael Fogden had lived in the cloud forest of Monteverde, Costa Rica, for nearly 15 years before he began to realize that gradually, subtly, the forest was changing. The signs were easy to overlook: Even at the height of the winter dry season, clouds still settled among the trees in this narrow zone of wet forest that straddles the spine of a mountain range 5,000 feet above sea level. Orchids, mosses and bromeliads still festooned the branches of the mist­shrouded trees, and every leaf and frond dripped with moisture. But for those who knew where to look, the signs were there. Many of the water­loving frogs that used to fill the forest had vanished. And as new species crept into the area, the fauna of Monteverde had grown to look more and more like that of drier habitats lower on the mountainside.

Through painstaking detective work, Fogden and an unlikely group of colleagues­­ - including biologists, climatologists and even an astrophysicist ­­may have discovered what´s behind these changes. Their findings indicate that as global climate change warms the nearby ocean, the cloud layer that gives the forest its name visits less and less often, especially at lower altitudes, leaving the flora and fauna thirsting for its life-giving mists. If they´re right, the cloud forests of Central America could be among the first entire ecosystems to fall casualty to global warming.

Fogden didn´t set out to study global climate change. An Oxford-trained ornithologist, he had walked away from his university career in the late 1970s so that he and his wife, Patricia, could start a new life as natural history photographers and writers. They settled in Monteverde, Costa Rica, a small community in the cloud forest. "We´ve become tropical derelicts," Fogden jokes. But old habits die hard, and he began keeping count of the birds of the forest year after year.

By the early 1990s, he was noticing that blue­crowned motmots, brown jays, golden­crowned warblers and other birds of drier, lower-elevation rain forests had begun nesting in his study area. He had even seen keel­billed toucans, also from lower elevations, breeding side ­by­ side with the symbol of the cloud forest, the resplendent quetzal. In all, Fogden´s notes meticulously document the arrival and breeding of 15 new bird species from downslope, plus others that have moved in but have not yet begun to breed­­a rate of nearly two new invaders per year.

Most of the original cloud forest birds are still there, too, though a few high­ elevation species that once were common, such as the fiery­throated hummingbird, the collared redstart and the ruddy treerunner, now show up rarely if at all. But Fogden expects a lot more species to vanish from his study area in the years to come. "Most cloud forest birds are long­lived and territorial," he explains. "It´s easier to stick to their territory even though it´s not as good as it used to be." But when the current occupant dies, no other member of its species may be willing to occupy a territory that´s no longer high­ quality cloud forest.

Nor are birds the only ones following such patterns. One lowland squirrel species, for example, had never been seen in Fogden´s study area until 10 years ago, only in drier, lower parts of Monteverde. Now, people see the squirrels every day in the study area. The same is true for the brilliant blue morpho butterflies. "They were very rare when we first arrived," says Fogden. "Now you can see them any sunny day."

Meanwhile, Fogden´s neighbor Alan Pounds, an ecologist and long­time Monteverde resident affiliated with the Monteverde Cloud Forest Preserve, had his eye on another problem. In 1987 alone, 20 of 50 frog species had vanished from his study area near Monteverde, and other extinction episodes came in 1994 and 1997. Most of the missing species have not returned. "If you walk through the forest, you don´t see any frogs. You used to see lots," says Pounds.

One lost species, the golden toad, was especially close to his heart, since it was found nowhere else on Earth. (Pounds´ e-mail moniker is "goldtoad.") Since few things are more important to amphibians than water, he began looking at weather patterns during the disappearances. Sure enough, the crash years had been drier than normal.

But in the cloud forest, as elsewhere, overall rainfall levels always vary from year to year. What could have been so special about these particular drier­than­normal years to cause such havoc among the frogs? "I kept thinking about the cloud bank, because it plays such a key role in the ecology here," says Pounds. But no one had kept records of where the cloud layer was over the years.

Then Pounds found a way to indirectly track the cloud layer. John Campbell, a meteorologist who also lives in Monteverde, had been keeping daily records of temperature and rainfall since 1973 at a weather station on Pounds´ study site. Though Campbell had not been specifically noting the height of the cloud bank, a misty day always produces measurable rainfall. On days where no rainfall was recorded, then, the researchers could assume that the cloud was not blanketing the forest.

Here was the key to the mystery. The number of dry days in each winter´s dry season increased steadily from 1973 to 1998, and so did the frequency of serious dry spells lasting five or more days. "It used to be that between cold fronts you still had some mist coming in, so that dry seasons were never as dry back in the 1970s as they are now," says Pounds. "Now when there´s no cold front, you get periods without mist." The drying trend wasn´t caused by El Niño­­a periodic disruption of ocean­ atmosphere systems that is a notorious cause of droughts ­­because it persisted even in non­El Niño years.

But a puzzle remained. Campbell´s temperature record showed a trend toward cooler days and warmer nights - ­­a sign that recent years were, if anything, cloudier than in the past. "It´s an interesting paradox that it would be getting cloudier and drier," says Pounds. They could think of only one explanation - ­­the cloud bank had shifted upward so that it drifted above the mountains on some days, blocking out the warming sunshine but not descending low enough to shroud the trees and mist up the forest.

That trend could explain why Fogden was seeing so many dry­country birds moving into Monteverde´s cloud forest. And lizards seemed to be responding in similar ways. Between 1983 and 1996, Pounds watched two cloud forest lizard species vanish from his study site. In contrast, a third species that also lives in the drier forest downslope continued to flourish in the cloud forest, possibly even increasing in number.

The drying may also explain what happened to the amphibians. Each of the three major population crashes came during a winter when dry days were especially common, Pounds found. One theory is that the creatures may also have suffered from a fungus that was discovered in 1998 to be strongly implicated in a worldwide die­off of frogs. But there may be a deeper cause at work, too. "Fungus is typically not an infectious disease," says David Wake of the University of California at Berkeley, who is one of the world´s leading experts on frog declines. "My guess is that generalized stress of some sort is giving these fungi a way in. An element of this stress could be the climate change that Pounds has found."

Meanwhile, completely unaware of all the work Pounds and Fogden were doing, climatologist Stephen Schneider of Stanford University had also begun pondering climate change in Costa Rica´s cloud forest. Schneider´s interest had been aroused in 1995, when his wife, ornithologist Terry Root of the University of Michigan, took him on a bird­watching holiday to Monteverde for his fiftieth birthday. While wandering through the cloud forest in search of the resplendent quetzal, he noticed the sharp ecological differences between that habitat and the drier one just down the mountain, and he was struck by the importance to the ecosystem of the moisture deposited by the mists.

On the flight home, an idea germinated. "Just as the airplane breaks above the cloud, this thing snaps in my head," he recalls. "What happens if the cloud deck lifts off?"

Back home, he and graduate student Christopher Still began thinking about the problem. Most global warming scenarios predict warmer sea surface temperatures off the Pacific coast of Central America, which should lead to warmer winds blowing onto land. Their temperature would carry them higher up the mountain before they cooled enough to form clouds. "That suggests the cloud deck should go up," says Schneider. But evaporation from the warmer ocean also adds more moisture to the system. "That suggests the cloud deck should go down; who´s going to win?" Schneider asks.

Together with Prudence Foster, an astrophysicist looking for a worldly line of research, Schneider and Still revved up a massive computer simulation of the Earth´s climate that Schneider had helped develop several years earlier. Once the computer had cranked through all the complex interrelationships that determine climate, it announced that on balance, the warmer temperatures would win out over the moisture as a deciding influence on the cloud cover. As global warming took hold, the climatic conditions that make cloud forest possible would shift up the mountains, squeezing the cloud forest into smaller and smaller remnants until the cloud lifted off the mountaintops altogether.

Unbeknownst to one another, Pounds´ and Schneider´s teams had stamped out opposite sides of the same coin. One was the theoretical prediction that as increased carbon dioxide in the atmosphere warms the globe and specifically the waters off Costa Rica, the cloud layer should climb upslope or even lift off the ridgetops. The other was the real­world observation that the phenomenon was indeed happening.

The two groups found one another quite by accident at a meeting in 1997 concerning the effects of climate change on wildlife. Both Schneider and Pounds spoke at the meeting, and they rapidly realized that each knew the missing parts to the other´s story. "We were forecasting what Alan had spent 20 years of his life observing," says Schneider. "It was thrilling."

The two halves of the story fit together to make a stark warning. Many climatologists think that high­ elevation tropical ecosystems­­ where a few degrees´ change can make the difference between freeze and thaw ­­should be among the first places to show the effects of global climate change. Indeed, other climatologists have already seen that glaciers atop the highest tropical peaks have been receding quickly in recent decades. By showing that warming may already be upsetting the delicate balance of the cloud forest near Monteverde, Fogden and Pounds are sounding an ecological alarm.

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Even so, Pounds, Schneider and their colleagues say they have more work to do to really nail down the connection between global warming, shifting cloud layers and ecological disruption. After all, Monteverde is just one small site in a large world. Would the same kinds of changes happen on other tropical mountainsides? The climate simulation suggests they might. When Foster repeated her analysis for three other cloud forests­­ at Serrania de Macuira in Colombia, Mount Kinabalu in Borneo and the Virunga Mountains in Africa­­ she found that warming produced a similar, though slightly less dramatic, lifting of the cloud layer in every case.

Unfortunately, real­ world confirmation at those sites and others may be hard to come by, because few tropical ecologists will have the good fortune to find such long­term weather data and population censuses from a single location. "It was sheer serendipity that three people set up study areas all on this one tropical mountain," says Pounds of the various Monteverde findings. Sometimes luck is the best teacher.

California writer Bob Holmes is an editor for New Scientist magazine.

Photographers Michael and Patricia Fogden have chronicled Costa Rica´s Monteverde cloud forest for a quarter of a century.